STL Algorithms

Algorithms operate on iterator ranges rather than owning containers. This separation lets one implementation work with arrays, vectors, lists, and custom ranges when their iterator capabilities satisfy the operation.

#include <algorithm>
#include <numeric>
#include <vector>

std::vector<int> values{5, 1, 4, 1, 3};
std::sort(values.begin(), values.end());

auto newEnd = std::remove(values.begin(), values.end(), 1);
values.erase(newEnd, values.end());

int total = std::accumulate(values.begin(), values.end(), 0);

std::remove moves retained elements and returns a new logical end; it does not resize the container. C++20’s std::erase/std::erase_if simplify common removal.

Predicates and transformations

Use find, find_if, count_if, all_of, any_of, and none_of for queries. transform maps values, copy_if filters, and for_each performs an action. Sorting requires random-access iterators; list supplies its own sort. Binary search algorithms require a range already ordered by a compatible comparator.

A comparator must establish strict weak ordering; returning left <= right is wrong because an element would compare before itself. Prefer pure predicates with explicit captures. <ranges> algorithms accept ranges directly and projections can select a member without a custom lambda. Know complexity: sorting is O(n log n), linear search O(n), and binary search O(log n) comparisons on a suitable range. Named algorithms make intent reviewable and often enable library optimization.